An experimental study of the antineutron-light nucleus annihilation at 1.4 GeV/c

Abstract

The heavy liquid bubble chamber, Gargamelle, has been used to investigate the gross features of the annihilation of antineutrons in light nuclei (mainly Carbon). Results are presented on the distributions of multiplicity, momentum and emission angle for charged pions and protons together with a simple model to describe them, These results may be useful for acceptance calculations for counter experiments detecting antineutrons by their characteristic annihilation.-2 -In this paper we describe the results of an experiment performed to determine the gross featuresof the 0-light nucleus annihilation process. The distributions of i) multiplicity, ii) emission angle and iii) momentum have been determined for both charged pions and protons together with some correlations.The experiment was performed in the large heavy liquid bubble chamber Gargamclle. magnetic The chamber, a cylinder of radius 1 m and length 4 m, situated in a field of 1.97 T, was filled with a propane-freon mixture of density -3 0.597 g em •The contributions of free protons, light nuclei (C, F) and heavy nuclei (Br) acting as targets for annihilation in this mixture are given in Table I. A beam of antiprotons at momentum 1.6 GeV/c entered almost along the axis of the chamber, and approximately 250000 pictures, containing on average three antiprotons were taken. This paper is organized as follows. In section 1 l'lle describe the properties of the incident antineutron "beam". In section 2 we give a general description c: our event sample. In section 3 we present the results. In section 4 we devel, a simple intranuclear cascade type model for the description of the data and in section 5 we give conclusions. 1) Properties of the n "beam"The antineutrons which were used for our investigation were produced in the charge exchange reaction pp -> on which was observed together with the subsequent annihilation ilA ·> all types of events.The data presented here are based on the full measurement of 484 cv••.nts of this type. To exclude U coming from other reactions we demanded at the scanning stage to see the charge exchange vertex together with the downstream antineutron star.The annihilation star was further required to be separated from the charge exchange vertex by at least 5 mm on the projection table (corresponding to ~ 2cm in space), The background due to the reactionswas excluded by the requirement of no 'I rays pointing to the charge exchange vertex. This removes .c, 85% 1 ) of the single 11° events and as the. cross section -3 -for charge exchange with the production of one neutral pion is already small, the residual background due to this reaction is negligible. The probability of seeing at least one y-ray for the reaction involving 2trG's is 98% which excludes possible background due to the charge exchange reaction "''ith many neutral pions.The distribution of our measured n production angles is shown in Fig. I.It peaks in a narrow forward cone as expected for charge exchange reactions.From two particle kinematics the energy of the ii. is...